1. Field of the Invention
The present invention relates to a contact of a vacuum interrupter and to a manufacturing process therefor.
2. Description of the Prior Art
Generally, a contact of a vacuum interrupter should consistently meet the following requirements:
(i) high large-current-interruption capability, PA1 (ii) high dielectric strength, PA1 (iii) high small-current interruption capability, PA1 (iv) low chopping current level, PA1 (v) excellent anti-welding capability, and PA1 (vi) low electrical resistance.
However, contacts of vacuum interrupters which can consistently meet all the above requirements are not yet available, given the present state of the art.
For example, U.S. Pat. Nos. 3,246,979 and 3,596,027 disclose, as a contact for a vacuum interrupter of magnetically arc-rotating type, a contact made of a Cu-0.5Bi alloy (hereinafter referred to as a Cu-0.5Bi contact) in which copper contains 0.5% by weight bismuth as a minor constituent with a high vapor-pressure and a low melting point.
A vacuum interrupter with a pair of Cu-0.5Bi contacts exhibits high large-current-interruption capability, excellent anti-welding capability and low electrical resistance, but remarkably low dielectric strength, particularly, a low dielectric strength immediately after a large-current interruption and a chopping current with a level as high as 10 A, so that it is susceptible to chopping surge during a current interruption. Thus, the interrupter can only poorly interrupt small-currents, particularly, inductive small-currents, which tends to lead to dielectric breakdown of electrical devices in inductive load circuits connected to the interrupter.
In addition, Japanese examined patent application publication No. 53-6710 and Japanese unexamined patent application publication No. 51-95291 disclose, as a contact for a vacuum interrupter which is designed to eliminate the drawbacks of the above-mentioned contact, a contact made of an Ag-WC alloy (hereinafter referred to as an Ag-WC contact) consisting of Ag and a material with a low vapor-pressure and a high melting point.
A vacuum interrupter with a pair of Ag-65WC contacts exhibits a chopping current with a level as low as 1.6 to 2.0 A but low large-current-interruption capability. An Ag-WC contact, which contains a relatively large amount of Ag, is expensive and has another drawback in that it is impossible to braze, particularly, to vacuum-braze at temperatures in excess of 950.degree. C.